1. Field of the Invention
The present invention relates to devices which have electrooptic light valve arrays for scanning light beams.
2. Description of the Prior Art
Electro-optic materials are those whose optical properties change in accordance with the strength of an electric field established within them. These materials make possible an electrically controlled "light valve array" which in this disclosure includes an electrooptic member which receives a monochromatic plane-polarized (linear) light beam, and has a plurality of pixel portions each of which changes the state of polarization of incident light from the beam in response to an established electric field. An analyzer receives light from the pixel portions and blocks light that passes through a pixel portion when its plane of polarization has not been changed (no electric field) while transmitting light when its plane of polarization has been changed by an electric field. The term "monochromatic" as used herein means having a narrow range of frequencies.
One example of an electrooptic material used in light valve arrays is lanthanum-doped lead zirconate titanate (PLZT). Although PLZT is a preferred electrooptic material, it will be recognized by those skilled in the art that other electrooptic materials can also be used to change the polarization of light.
Without an electric field being established, some compositions of PLZT are optically isotropic, while others exhibit a static birefringence. In either case, when an electric field is applied through a body of PLZT, the PLZT crystal structure changes. This change in crystal structure causes a change in birefringence. An optic axis is thereby formed which is aligned parallel to the electric field lines. The optic axis is a direction, and not just one particular line.
A plane-polarized light wave incident upon the surface of a birefringent material from a direction perpendicular to the optic axis of the material can be resolved into two orthogonal component waves, one plane-polarized normal to the optic axis and the other plane-polarized parallel to the optic axis. These component waves travel through the material at different speeds and on leaving the birefringent material into an isotropic medium, the phase relation between the waves will be altered.
When a plane-polarized light wave enters and exits a birefringent member normal to the entering and exiting surfaces of the member and perpendicular to its optic axis, the two component waves exit the member at the same location and neither is refracted. Thus, they again form one wave. The phase difference between the exiting waves depends on the difference in the indices of refraction of the member in the orthogonal directions, and the thickness of the member. If these quantities are such that in traveling through the member, one wave is delayed behind the other by a one-quarter a wavelength, the member is a quarter-wave plate; if the delay is one-half a wavelength, the member is a half-wave plate. A half-wave plate changes the direction of polarization of a beam of polarized light by 90.degree. provided the incident polarization direction makes an angle of 45.degree. with the optic axis of the half-wave plate. With a quarter-wave plate, plane polarized light is converted to circularly polarized light when the plane polarized light makes an angle of 45.degree. with the optic axis.
Light valve arrays have been constructed which include an isotropic member that has a pixel portion which can be converted into a half-wave plate by the establishment of an electric field. This member is sandwiched between crossed polarizers whose polarization orientations are typically at 45.degree. angles to the optic axis of the half-wave plate. A light valve array has a plurality of such half-wave plates.
One preferred light valve array configuration is shown in U.S. Pat. No. 4,229,095 to Mir, includes a light valve array having a member formed of electrooptic material, such as isotropic lanthanum-doped lead zirconate titanate (PLZT), and a plurality of selectively addressable electrodes which define pixel portions of such member. The member is sandwiched between crossed polarizers. A pixel portion transmits received monochromatic light. When its electrodes are energized and establish an electric field, the pixel portion becomes a half-wave plate and changes the plane of polarization of incident light by 90.degree..
Light valve arrays must address many image pixels per line at an imaging zone in order to form images having even moderate detail. The number of pixels per line at an image zone increases in accordance with the resolution requirements of the imaging application. For example, it may become as large as 250 pixels per inch or larger for high-quality continuous-tone imaging. With prior devices, each image zone pixel is addressed by its own pixel portion. It is a difficult task to build light valve arrays with this many independently addressable pixel portions per line inch of image zone pixels.
Commonly assigned U.S. Pat. No. 4,377,753 to Mir discloses a light valve array arrangement with improved optical resolution. Each pixel portion of the light valve array addresses not one but a plurality of pixels in a particular region at the image zone. The arrangement includes an acoustooptic deflector which, in response to a variable frequency signal, deflects received plane-polarized light. The frequency of the signal controls the deflection angle while the amplitude of the signal controls the beam intensity. The device also includes a linear lens array and a member, formed of electrooptic material, which has electrodes that define an array of pixel portions. Each lens in the lens array corresponds to an array pixel portion. As light is deflected or scanned by the acoustooptic deflector through different parts of a particular lens it is directed through different parts of its corresponding pixel portion. The light is focused by the lens at the image zone. Electrical signals are selectively applied to the electrodes of the pixel portion, causing the pixel portion to change the plane of polarization of its transmitted light beam by 90.degree.. An analyzer blocks those light beams whose planes of polarization have not been changed and transmits those whose planes of polarization have been changed.
Although this light valve array arrangement provides significant resolution improvement over previous arrangements, it is relatively bulky and difficult to align the optical elements. Another problem this array arrangement has is that it is a difficult process to electronically alter the acoustic drive frequency in an acoustooptic deflector to provide for highly accurate beam deflection which is needed in many scanning applications.